In recent years, standards for interconnecting electronic devices have been established. For example, as a standard that enables usage just by connecting a peripheral device to a computer (hereinafter “PC”), PnP (Plug and Play) is known.
By connecting a video display device and a PC, both of which support such a standard, a standard display characteristics can be obtained without a user having to install a driver software or make color adjustments. A video display device of this sort is described in Japanese Patent Application Laid-Open No. 07-302068-A (D1).
In D1, a communication line for communications between the PC and the video display device is provided, and through the communication line the PC reads identification information and timing information of the video display device, generates and outputs video having appropriate signal timings or in an appropriate signal format to the video display device.
DDC (Display Data Channel), issued by VESA (Video Electronics Standards Association), is a communication standard for communication between a video display device and a video output device. DDC is employed, for example, in such display interface standards as HDMI (High-Definition Multimedia. Interface) and DVI (Digital Visual Interface).
Video output devices that support DDC version 2 (DDC2) or higher, which supports bidirectional communication, once connection of a video display device has been detected by a connect detection signal, output a clock signal (up to 100 kHz) to the clock line (SCL line) used in DDC. Then, the video output device transmits an EDID (Extended Display Identification Data) transmission request command to the video display device. EDID is video display device attribute information, and it includes product information such as model name and manufacturer name as well as information relating to display capability (such as displayable resolution, vertical sync frequency combinations, and so forth) and characteristics (such as gamma values). Once the EDID transmission request command is transmitted to it from the video output device, the video display device immediately sends back the EDID in synchronization with the SCL line clock. The video output device then receives the EDID transmitted from the video display device and ascertains the video display device configuration, which it reflects in a process of generating the video that is output thereafter.
The connect detection signal is detected by the video output device by mechanical and electrical connection through an interface cable of the respective interface connectors of the video output device and the video display device.
Then, once the video output device detects the connect detection signal, communication with the video display device from clock output, the EDID transmission request command transmission and up to EDID reception is performed automatically. Although this automatic process makes it possible to use the video display device without the user having to perform special settings, conversely, it might be a problem that once the connect detection signal is detected, communications between the devices after that cannot be stopped.
For example, to conduct communications using DDC2, it is necessary to make sure that all the signal lines, including SDA (I2C), SCL (I2C), VCLCK, DDC5V, HPD (Hot Plug Detect), GND, and so on are connected correctly. However, because it is handled by the user, the connection between the interface connector on the device side and the interface connector provided at the end of the interface cable is not always connected correctly.
Thus, for example, in a case in which the interface cable connector is inserted at an angle to the device-side connector, even though the connect detection signal (HPD) pin might be in a connected state, the connections for the other pins, such as the SCL and SDA signal pins, may be incomplete.
If in such a state communication involving transmission of an EDID transmission request command or the like is started automatically, the video output device cannot correctly receive the EDID. In that case, despite the fact that the video display device is connected, the video output device might decide that the EDID contents cannot be read, and as a result stop video output.